1. Field of the Invention
The present invention pertains generally to the field of solid-state image sensor and more particularly is a CMOS Time Delay and Integration (TDI) sensor for X-ray image scanning applications.
2. Background of the Invention
This invention is related to Time Delay and Integration (TDI) CMOS linear image sensor suitable for high-speed X-ray image scanning applications. TDI image sensors are used in high-speed line scan applications where the integrated input light signal is very low. In normal line scan application, one way to increase the integrated input light signal is to reduce the scan speed and thus increase the integration time. The TDI sensor allows the line scan detector system to increase the light signal without sacrificing the scan speed. It is normally implemented using charge transfer device, such as charge-coupled device (CCD).
In a CCD TDI array, each detector pixel contains N stages of TDI locations. For example, for an M pixels linear detector, it will contain a two dimensional M by N stages CCD array. The N stages CCD for each pixel is in parallel to the direction of scan. In operation, the first stage of CCD integrates the light signal within one integration time which equals to one line time. The signal charge will then transfer from the first stage to the second stage of CCD while the object under scan also moves from the first stage to the second stage of CCD in synchronization with the movement of the signal charge. The second stage CCD will integrate signal charge during the second integration time for the same object. As a result, at the end of the integration time the signal charge at the second stage CCD will be twice the signal charge as compared with the charge it receives from the first stage. The signal charge of the second stage will then move to the third stage in synchronization with the object movement. Again, the third stage CCD integrates light signal in addition to the signal it receives from the second stage. The process repeats and when it reaches the final N stage CCD, the light signal is multiplied by N times. An output CCD shift register then reads out the M pixels signal in sequence.
Although CCD TDI imaging system has been used extensively in visible high-speed industrial inspection applications and medical X-ray scanning applications, such as CT scan and panoramic dental scan, it does have drawbacks in industrial x-ray inspection applications. In an industrial X-ray inspection system, the detector pixel size is normally quite big as compared with normal CCD sensor pixel size. The required pixel size in such an application ranges from a few tenth of millimeters to a few millimeters. As the pixel size increases, the CCD scanning speed decreases significantly and as a result, make it unsuitable for such an application.
A second drawback for a CCD TDI system is that it is very susceptible to X-ray radiation damage. In medical X-ray scanning applications, the X-ray energy and dose used is normally much lower than that of industrial inspection applications. In medical applications, not only the X-ray dose is regulated by Federal Drug administration (FDA), the energy is normally less than 100 KeV because of soft human tissue. For industrial applications, the energy used can range from 50 KeV to 15 MeV depending on what kind of material needs to be inspected. Since there is no regulation to limit the X-ray dose in an industrial inspection system, the dose can be much higher than that of medical scanning systems. The accumulation of radiation exposure of a CCD sensor under X-ray will increase its dark current, shift its well potentials, and, as a result, reduce its usable lifetime.
The current invention is to implement a CMOS detector system that alleviates the drawbacks of CCD detector in an industrial X-ray inspection system. Since signal charges can not move from one CMOS circuit to another CMOS circuit in charge domain, it is more difficult to implement a TDI sensor using CMOS circuitry. The present invention is a method to implement TDI sensor in CMOS circuitry using charge integrating and summing amplifiers.
Accordingly, it is an object of the present invention to provide a Time Delay and Integration (TDI) image sensing structure that can be implemented using standard CMOS manufacturing processes.
Another object of the present invention is to provide a TDI image sensing detector system that is suitable for bigger pixel-to-pixel pitch, such as in an industrial X-ray detector system, without sacrificing the readout speed.
A further objective of the current invention is to provide an X-ray TDI detector system that the CMOS circuitry can be separated and away from the photodiode detectors so that the CMOS circuitry can be easily shielded from X-ray radiation damage.